Notes

MessageBoxEx does not support reentrancy. Good thing too. Why would you want to have more than one message box at the same time?

If the user doesn't press a button before the timeout elapses the function will return the
DialogResult of the messagebox dialogs default button.

How does it work?

Just before calling MessageBox.Show(...), SetWindowsHookEx(WH_CALLWNDPROCRET, ...) is called. The hook proc looks for a
WM_INITDIALOG on a window with text equal to the message box caption. A windows timer is started on that window with the appropriate timeout. When the timeout fires
EndDialog is called with the result set to the dialog default button ID. You can get that ID by sending a dialog box a
DM_GETDEFID message. Pretty simple.

References

KB318804: HOW TO: Set a Windows Hook in Visual C#.NET

Acknowledgements

GipsySoft: http://www.gipsysoft.com/messagebox. You'll find a C++ implementation with many more features.